1. Field of the Invention
The present invention relates to a sound reproduction system and a sound reproduction method to be used in a stereo audio system, a television system, etc. In particular, the present invention relates to a sound reproduction system capable of conducting an automatic tone control for an audio signal.
2. Description of the Related Art
In recent years, audio visual systems have made rapid progress, requiring high quality sound reproduction, as well as wider screens of display devices and high quality image reproduction. As the sizes of display devices increase, opportunities for several people to enjoy movies, music, etc. at the same time have increased. This has led to the need for a sound reproduction system having uniform acoustic characteristics irrespective of the listening position of each listener.
Conventional devices for realizing uniform sound reproduction are disclosed, for example, in Japanese Patent Application No. 1-203374. FIG. 10 shows an exemplary configuration for such a conventional sound reproduction system 700.
As is shown in FIG. 10, the sound reproduction system 700 includes audio signal input terminals 101, a signal processing section 703, an in-phase adder 107, an anti-phase adder 108, amplifiers 110, a left (L) -channel loudspeaker 111, and a right (R)-channel loudspeaker 112. The signal processing section 703 includes an adder 104 for summing up the right and left channel signals, a low-cut filter 105, and a group delayer 106. The low-cut filter 105 may be either a high-pass filter or a band-pass filter. The group delayer 106 is required to be an all pass filter which has stable frequency characteristics of amplitude, only the delay time of the group delayer 106 being variable depending on the frequencies.
Audio signals SL701 and SR701, which are input through the audio signal input terminal 101, are added up by the adder 104. The adder 104 outputs an audio signal S702. The low-cut filter 105 receives the audio signal S702 and allows only a predetermined band thereof to be output as an audio signal S703. The group delayer 106 receives the audio signal S703 and subjects the audio signal S703 to a group delaying process, so as to output an audio signal S704. The delayed audio signal S704 is supplied to the in-phase adder 107 and the anti-phase adder 108, so as to be added with the input audio signals SL701 and SR701, respectively. The in-phase adder 107 and the anti-phase adder 108 output audio signals SL705 and SR705, respectively, to the amplifiers 110 so as to be amplified thereby. The amplified audio signals SL705 and audio signal SR705 are reproduced by, respectively, the L-channel loudspeaker 111 and the R-channel loudspeaker 112.
FIGS. 13A and 13B show transmission characteristics of the audio signals reproduced by the sound reproduction system 700, with respect to listening positions of the listener. In FIG. 11, which shows the conditions for the measurements of the transmission characteristics shown in FIGS. 13A and 13B, W denotes the distance between the loudspeakers 111 and 112; M denotes the middle point between the loudspeakers 111 and 112; R denotes a distance between the middle point M and a measurement point P.sub.1 or P.sub.2 ; X denotes a center axis originated from the middle point M; and .theta. denotes an angle between the measurement point P.sub.1 or P.sub.2 and the center axis X.
FIGS. 13A and 13B each show transmission characteristics taken at a range of measurement points from P.sub.1 (located on the center axis X; i.e. .theta.=0.degree.) to P.sub.2 (.theta.=30.degree.), under the conditions that W=80 cm and R =240 cm. The input signals are monophonic signals. FIG. 13A shows the transmission characteristics when the input audio signals are not processed by the signal processing section 703. FIG. 13B shows the transmission characteristics when the input audio signals are processed by the signal processing section 703. As is seen from FIG. 13A, when the input audio signals are not processed by the signal processing section 703, the transmission characteristics have a large dip caused by the phase interference between the signals from the right loudspeaker 112 and the left loudspeaker 111. This is because the difference between the distances of the measurement point P, taken from the L-channel loudspeaker 111 and the R-channel loudspeaker 112, becomes larger as angle .theta. increases. This dip in the transmission characteristics can be corrected, as is shown in FIG. 13B, by processing the input audio signals in the signal processing section 703, and specifically by adding the delayed signals from the group delayer 106 to the input audio signals so as to be output from the loudspeakers 111 and 112.
FIGS. 12A and 12B schematically show impulse responses measured at P.sub.1 and P.sub.2, respectively. At each of the measurement points P.sub.1 and P.sub.2, respective impulse waveforms reproduced by the right loudspeaker 112 and the left loudspeaker 111 combined. As a result, the delayed signals cancel each other at P.sub.1, while they function to compensate the distortion in the transmission characteristics at P.sub.2. Thus, as is shown in FIG. 13B, uniform acoustic characteristics are realized irrespective of the listening position of a listener, as long as monophonic signals are reproduced by means of the right loudspeaker 112 and the left loudspeaker 111.
The performance of a sound reproduction system can be estimated with another factor: the articulation score, which indicates how clearly a reproduced voice can be heard by a listener.
FIGS. 14A and 14B each show monosyllable articulation scores examined in a range of measurement points from P.sub.1 (.theta.=0.degree.) to P2 (.theta.=30.degree.), under the conditions that W=80 cm and R=240 cm. The input signals are monophonic voice signals. A monosyllable articulation score is defined as a percentage representing how many meaningless monosyllables that are clearly pronounced in a random sequence, e.g. "ka", "sha", "pa", "re", and so on , are accurately recognized by a listener, taken against the total number of such clearly pronounced monosyllables.
FIG. 14A shows the results obtained when the input audio signals are not processed by the signal processing section 703. FIG. 14B shows the results obtained when the input audio signals are processed by the signal processing section 703. As is seen from FIG. 14A, when the input audio signals are not processed by the signal processing section 703, the articulation score decreases as the listening position shifts from the center axis X. When the delayed signals from the group delayer 106 are added to the input audio signals, as is shown in FIG. 14B, a high articulation score is obtained irrespective of the listening positions.
As is described above, a uniform speech articulation score, irrespective of the listening positions, is realized by processing the input audio signals to compensate for a decrease in the speech articulation that occurs at a listening position offset from the center axis X. irrespective of the listening positions, in cases where the input audio signals are monophonic signals and are reproduced by means of right and left loudspeakers.
FIG. 15 shows a configuration for a conventional sound reproduction system 800 for conducting a tone control. The sound reproduction system 800 is to be used in a television system. A monitor system for reproducing image information is omitted in FIG. 15. An operation of the sound reproduction system 800 is described with reference to FIG. 15 below.
First, a broadcast wave is received by an antenna 404. An audio signal S801 is extracted in a decoder 405. A filter 801 adjusts the tone of the extracted audio signal S801 so as to output a signal S802. The signal S802 is reproduced by a loudspeaker 406. A listener (viewer) 407 switches a remote control switch 402 depending on the kind of broadcast wave. In accordance with the mode of the remote control switch 402 switched by the listener 407, a CPU 803 controls the filter 801 so as to have a corresponding pattern of amplitude-frequency characteristics. The filer 801 is usually referred to as a tone controller, or a bass/treble, which is a filter composed essentially of operational amplifiers and specific ICs for modifying the amplitude characteristics in low frequency bands or high frequency bands.
Each of FIGS. 16A to 16D shows amplitude-frequency characteristics of the filter 801, the vertical axis of the coordinates indicating amplitudes, and the horizontal axis indicating frequencies. Specifically, FIGS. 16A to 16D correspond to the respective frequency characteristic curves selected by means of the remote control switch 402. The characteristics curve C1 (FIG. 16A) shows amplitude-frequency characteristics in which the low frequency bands are boosted; the characteristics curve C2 (FIG. 16B) shows amplitude-frequency characteristics in which the low frequency bands are attenuated; the characteristics curve C3 (FIG. 16C) shows amplitude-frequency characteristics in which the high frequency bands are boosted; and the characteristics curve C4 (FIG. 16D) shows amplitude-frequency characteristics in which the high frequency bands are attenuated.
However, according to the conventional techniques described above, the listener is required to manually perform selective processing of an input audio signal, depending on whether the input audio signals are monophonic signals, stereophonic signals containing a monophonic signal component (i.e. a signal component located in the center), or completely stereophonic signals. The reason is that, according to the sound reproduction system 700 of the above-described configuration, a stereophonic input signal is also subjected to the same signal process that a monophonic signal is subjected to. The signal process conducted for the stereophonic signals, which is meant to realize uniform acoustic characteristics, results in distortion of the transmission characteristics, because of the phase interference between the signals reproduced from the right and left loudspeakers.
Further according to the conventional techniques, the listener is also required to manually perform a selective processing of an input audio signal in cases where the input audio signal contains a voice signal component. This is because the same signal process is conducted for the non-voice signal component as well as the voice signal, thereby disadvantageously letting the sound image be dispersed instead of compensating the distortion to increase the speech articulation as is intended.
Moreover, according to the sound reproduction system 800 of the above-described configuration, when tone control is conducted so as to boost and/or attenuate the low frequency bands or the high frequency bands as is shown in FIGS. 16A to 16D, not only the frequency characteristics but also the total sound volume of the input signal is changed. In order to conduct the tone control while maintaining the total sound volume of the input signal at the same level, it is necessary to adjust the total sound volume concurrently with the tone control. In addition, the tone control can only be achieved by a manual operation of the listener, which is conducted in accordance with the kind of broadcast wave by means of a remote control switch.